Thyroxine-binding globulin (TBG)

Thyroxine-binding globulin (TBG) is a glycoprotein primarily synthesized in the liver that serves as the main high-affinity transport protein for thyroid hormones (T4 and T3) in the blood. While TBG is not a direct therapeutic target, its serum concentration is crucial for interpreting thyroid function tests. Changes in TBG levels, influenced by factors such as pregnancy, certain medications, and genetic variations, alter total circulating thyroid hormone levels without necessarily affecting free hormone levels or clinical status. Inherited TBG deficiencies are usually benign but diagnostically important to avoid misinterpreting thyroid function tests.

Other names

Serpin family A member 7TBG

TBG levels are affected by several drugs. Estrogens/oral contraceptives increase TBG production, and androgens/anabolic steroids/glucocorticoids decrease TBG production. This, in turn, affects the total T4/T3 levels but typically not the free T4/T3 levels.